EPUB changes.

* Epub writer now exports writeEPUB, not writeEPUB2 and writeEPUB3.
* Options now exports EPUBVersion
* WriterOptions now includes writerEpubVersion.
For epub3, use epub:type to mark footnotes and footnote refs.

This yields nice popup footnotes in iBooks.  See
http://www.pigsgourdsandwikis.com/2012/05/creating-pop-up-footnotes-in-epub-3-and.html